Recently, in the field of printers, impact printers have been rapidly replaced by non-impact printers such as ink jet printers, which can more easily be adapted to color printing and multiple gradations. Among ink jet heads used for ejecting ink in this connection, those of drop on demand type where only droplets required for printing are ejected are of particular interest because they provide a highly efficient ejection and allow an easy cost reduction. Common drop on demand printers include Kyser printers and thermal jet printers.
The Kyser printer, however, has a drawback of being difficult to be made smaller and more compact. The thermal jet printer is suitable for a more compact design. However, it requires the ink to be heat-resistant because it has a heater for heating ink to produce bubbles in the ink, whose energy is used to cause ejection. In addition, the heater cannot easily achieve a long life, and has a low energy efficiency, resulting in a large power consumption.
In order to solve their respective problems, ink jet printers have been disclosed that utilize the shear mode of the piezoelectric member. This type of printers use electrodes provided on both sides of the wall between ink channels (hereinafter referred to as “channel walls”) made of a piezoelectric to produce an electric field perpendicular to the polarization of the piezoelectric, thereby deforming the channel walls by virtue of the shear mode, which causes variations of pressure wave that is used to eject ink droplets. This type of printers are suitable for producing a more compact nozzle, reducing power consumption and increasing driving frequency.
Referring to FIG. 12, the structure of such an ink jet head using the shear mode will be described. The ink jet head includes a base 1 having therein a plurality of channel grooves 4 that is made of a piezoelectric material polarized in the vertical direction in FIG. 12, a cover 2 having an ink supply port 21 and a manifold space 24, and a nozzle plate 9 with nozzle orifices 10, all of which are bonded together to provide ink channels. “Ink channels” are pressure chambers that are provided by the inner space of channel grooves 4. Each of channel walls 3 is provided with electrodes 5 on its upper half only, for allowing the application of an electric field. The side of an ink jet head with nozzle plate 9 is hereinafter referred to as “front”, the opposite side as “rear”. In the above ink channels, the rear end of each channel groove 4 has been machined to form an arc shape corresponding to the diameter of a dicing blade used for forming the grooves, and also has a shallow groove portion 6, which is an electrode lead portion for electrical connection with the outside and is provided by, again, a dicing blade machining. At the rear end of shallow groove portion 6, the electrode in shallow groove portion 6 is connected to an outer electrode 8 on a flexible printed board 11 via a bonding wire 7. An ink jet head with such structure is supplied with ink from manifold space 24 via the arc shaped region, where the required pressure for ejection is produced in the region in which the top of each channel wall 3 in base 1 is bonded to cover 2. The arc shaped region is not used and yet adds capacitance.
An ink jet head without an arc shaped region for decreasing capacitance is disclosed in Japanese Patent Laying-Open No. 9-94954. The disclosed ink jet head, however, has a connection on the bottom surface of the base board for connecting the electrodes on the channel walls to the outside, requiring a complicated process to provide connecting electrodes.
Thus, ink jet heads with decreased capacitance and with an easier connection of the electrodes on the channel walls to the outside are proposed as shown in FIGS. 13 and 14. FIG. 13 is an exploded perspective view of such an ink jet head. FIG. 14 is a cross sectional view of the assembly thereof The ink jet head is characterized by channel grooves 4 that penetrate from the front end to the rear end of base 1 with a constant depth. This structure does not have an arc shaped region such that the capacitance is decreased. Also, the amount of piezoelectric material used can be reduced. Close to the rear end of each channel groove 4, channel groove 4 is sealed by a conductive resin 26 to provide an electrical connection such that those electrodes 5 that face the same one of channel grooves 4 are maintained at the same potential. Conductive resin 26 extends to the rear end of its respective channel groove 4, and the rear end of base 1 is connected to a flexible printed board 11 with an interposed anisotropic conductive film (hereinafter referred to as an “ACF”) 12. An outer electrode 8 on the surface of flexible printed board 11 is electrically connected to conductive resin 26 by sandwiching ACF 12 therebetween and pressing it in the direction of its thickness. Nevertheless, the properties of ACF 12 keep each ink channel to be electrically independent.
In general, an ACF need to be pinched with a pressure greater than a predetermined level in the direction of its depth in order to establish electrical connection. In the ink jet head described above, an outer electrode 8 is electrically connected to a conductive resin 26 by pinched ACF 12. In reality, although channel groove 4 is plugged with conductive resin 26, the top of each channel groove 4 is open even at the rear end of channel groove 4 such that pinching ACF 12 and pressing flexible printed board 11 onto base 1 causes conductive resin 26 to move away toward the front. This may prevent conductive resin 26 from pressing ACF 12 with a sufficient pressure, resulting in increased electrical resistance between conductive resin 26 and outer electrode 8, which should be electrically connected to each other, or in unstable connection.
Moreover, conductive resin 26 has a coefficient of linear expansion greater than that of piezoelectric materials used for base 1. As a result, a variation in temperature may produce a crack between a conductive resin 26 and a channel wall 3 adjacent thereto.